1.3. Structural features associated with continental rifting
This section lists the major tectonic structures and basin types encountered in rifts and rifted margins. Basic definitions and schematic diagrams are provided to clarify the main terminology used in the following chapters of this book and to avoid any misunderstanding.
Note that this section does not aim to describe the details of all structural features that can be encountered in extensional settings. The descriptions below cover a biased and non-exhaustive selection of structures that are considered to be representative of rifts and rifted margins. For a proper primer in structural geology, the reader is referred to contributions specifically dedicated to this theme (see the Further reading sections, and, for example, Fossen 2010).
1.3.1. Extensional mechanisms
The lithospheric mechanisms that accommodate extension can be roughly divided into three categories: pure shear, simple shear and crustal flow (Figure 1.13). Pure shear refers to a homogeneous flattening of a rock body that is elongated in one direction and shortened perpendicularly. First applied by McKenzie (1978) to regional rift settings, this mechanism assumes that the lithosphere deforms uniformly. Simple shear is a deformation mode involving a rotational component, in which parallel markers in a rock body remain parallel while translating relative to each other. (see Chapter 2 for further description of the models applied to rift settings). Numerical, analogue and conceptual models have been developed and tested based on these core mechanisms. Even though each deformational mechanism is based on an idealized version of the lithospheric structure and rheology, they are considered to represent most of the structural settings encountered on Earth.
Figure 1.13. Illustration of the pure shear and simple shear deformation mechanism (source: modified after Fossen 2010)
1.3.2. Main structural geometries
The following list contains most of the deformation structures that characterize extensional settings. Schematic figures are provided to relate each structural geometry with field and seismic examples in order to illustrate their typical identification and interpretation in rifted margin studies.
1.3.2.1. Normal faults
In geology, faults correspond to discontinuities in rocks along which displacement occurs: when a rock mass is submitted to forces, discontinuities may develop. Depending on various parameters of the rock (e.g. rigidity, rheology), these discontinuities can accumulate a certain amount of stress. When the stress exceeds the strength limit of the rock, the discontinuity can rupture, creating a fracture, and strain energy is released. Depending on various parameters, such as the rheology of the rock (brittle, ductile), pore pressure, temperature and strain rate (low, high), the strain release can be gradual or instantaneous.
Fractures can be of various sizes (shear fractures, joints, faults, detachment faults) and geometries (high angle, low angle, listric or spoon-shaped, concave upward/downward), with large or little to no movement, and accommodate various types of displacements (normal, reverse, strike-slip and oblique-slip). The two blocks separated by the fault plane are called the hanging wall (above) and footwall (below).
The Scottish geologist Ernest Masson Anderson (1877–1960) introduced the basic definitions of fault mechanics (Anderson 1905), using the Mohr–Coulomb theory to explain the different dips of the various types of faults. He divided faults into three principal types (normal, reverse, strike-slip) depending on the orientation of the compressive stress axes (Figure 1.14): in an idealized earth, if σ1 – the maximum principal compressive stress – is vertical, the crust is extended, and normal faults are generated at a high angle (dip around 60°). When σ1 is horizontal and σ3 – the lowest principal compressive stress – is vertical, shortening occurs and reverse faults are generated at a low angle (dip around 30°). If both maximum and minimum principal stresses are horizontal, or tangential to the Earth’s surface, and σ2 – the intermediate principal stress – is vertical, then near-vertical strike-slip faults are generated. The angle between the σ1 axis and the shear plane is called the angle of internal friction (α). This relationship between faulting and stress is known as Anderson’s theory.
Depending on the overall geometry of the fault, additional terms can be used such as: 1) thrust faults are reverse faults with a dip under 45°; 2) listric faults are normal faults with a fault plane that curves with depth and flattens into a sub-horizontal layer often called decollement; 3) synthetic and antithetic faults describe minor or secondary faults associated with a major fault: synthetic faults dip in the same direction as the primary fault, whereas the antithetic fault dips in the opposite direction.
Figure 1.14. Illustration of the three ideal Andersonian fault types. S refers to the principal stress axes
Faults are actually complex structures, and it is often impossible to define a single fault surface for the fault plane, so interpreters regularly use the term fault zone to refer to the complex deformation associated with multiple fault surfaces. The fault core is where most of the displacement has been accommodated, a damage zone, where the rock is highly deformed and a drag zone, with kinematic indicators that show the amount and/or direction of displacement (Figure 1.15) (e.g. Fossen 2010).
Faults are described by their length (L) and their displacement (D) (Figure 1.16) (Elliott et al. 1976; Watterson 1986; Scholz 2019), and various questions remain on the displacement–length relationships and fault growth mechanisms: faults rarely originate with their total length extent. This is the reason why we talk about “fault nucleation”. The rock block separation is supposed to initiate on a nucleus and extend in a certain direction. However, the mechanism and time–space evolution are not fully understood. In terms of normal faults in rifts, fault growth models can be divided into two categories: (1) the “propagating fault” or “isolated fault” model and (2) the “constant-length” fault model (Rotevatn et al. 2019). In the propagating fault model, the displacement synchronously increases and accordingly the fault grows in length (Cartwright et al. 1995; Walsh et al. 2003). On the other hand, with the constant-length model, the fault is supposed to reach almost its full length extent early in its evolution, and displacement occurs at later stages by successive increments (Jackson and Rotevatn 2013; Childs et al. 2017). Based on case examples, Jackson et al. (2017) show that normal fault growth may incorporate both models, with a predominance of the constant-length model and periods of minor fault-tip propagation and coeval displacement accumulation.